Cell lines in cultures derived from mouse teratocarcinoma are used to investigate cellular differentiation in mouse embryo as well as cancer development. Syngeneic antisera have been produced in mouse 129 males against the primitive cells of teratocarcinoma (PTC). These sera react specifically with the primitive cells and are negative on various types of differentiated teratoma cells. They are negative on all other mouse cells listed with the exception of spermatozoa and cleavage stage embryos. The origin, biochemical structure, genetic determinants and distribution among embryonic cells of this cell surface antigen(s) involved in embryonic development are now under investigation. Other cell lines are being derived from teratomas to investigate other cell surface antigens acting at other stages of development. PTC cells capable of differentiation multiply and form tumors in isogenic but not in allogenic mice where they are rejected. However, they can multiply in allogenic mice whose defense mechanism is decreased for a short while after cell injection. The properties of the cells and of the early immune mechanism of the host during the initial steps of tumor formation are under investigation.